1837, from un- (1) "not" + mentionable (adj.). Humorous use of unmentionables "trousers" is attested from 1823; from 1910 as "underwear," both on notion of "articles of dress not to be mentioned in polite circles."

early 15c., "not in accord with physical nature," from un- (1) "not" + natural (adj.). Meaning "artificial" is attested from 1746; that of "at variance with moral standards" is from 1520s. Related: Unnaturally; unnaturalness.